In wireless communication, radio frequency (RF) receivers are used in a wide variety of applications such as television, cellular telephones, pagers, global navigation satellite system (GNSS) receivers, cable modems, cordless phones, satellite radio receivers, and the like. As used herein, a “radio frequency” signal or RF signal is an electrical signal conveying useful information and having at least one oscillation frequency, regardless of the medium through which the signal is conveyed. Typically the useful information of an RF signal is carried on top of an RF carrier frequency and needs to be converted to base band so that the information contained therein can be more conveniently extracted. Radio receivers are devices utilized to achieve this goal. One type of radio receiver is a super-heterodyne receiver, in which an incoming RF signal, received by an antenna, is mixed with a signal from a local oscillator (LO) to produce a mixed lower frequency signal called an intermediate frequency (IF) signal. The IF signal is then further down-converted in one or more additional steps, before being amplified, filtered, and converted to a base band signal. The base band signal can be demodulated and provided to a detector that extracts information conveyed by the received RF signal.
In a conventional super-heterodyne receiver having one or more stages of down-conversion, a local oscillator (LO) is used to mix the incoming radio frequency (RF) signal, generating sum and difference frequencies. A “real” mixer is often used in one or more stages of the down-conversion. One problem with mixing using a real mixer is the presence of image signals. When the real mixer is used to shift an RF signal either up or down the frequency spectrum as part of a mixing process, other spectral components that lie within the bandwidth of the mixer may be shifted as well. A special case of such problematic spectral components is termed image frequencies. Image frequencies form an image band that extends from the LO frequency on opposite sides of a desired band. Both the image and desired frequency bands may be down-converted to the same lower frequency as a result of the mixing process in the real mixer, unless some means is provided to prevent it.
In some RF systems, the level of the image signals is small enough that designers can rely on attenuation characteristics of an RF band-pass filter alone to reject the image band. In other systems, the attenuation provided solely by the RF band-pass filter is not sufficient. One known image rejection method uses a notch filter to reject the image band before the RF signal enters the mixer. The center frequency of the notch filter is set to track the local oscillator frequency so that signals having frequencies in the image band are attenuated.
A designer may choose the IF signal to have a relatively high intermediate frequency, thus providing more separation in the bandwidth between the image band and the desired band, and making it easier for a notch filter to reject image band signals. However, choosing a higher intermediate frequency comes at a cost, such as an increase in power consumption in succeeding stages of receiver circuits. Furthermore, designing circuits to operate at a higher frequency is more demanding and consequently not desirable. Alternatively, a designer may select a relatively low intermediate frequency, which in turn necessitates a higher performance notch filter. However, fabricating and integrating high-performance filters in RF integrated circuits can be difficult and costly. For example, high-performance notch filters, such as SAW filters, cannot be integrated on-chip. Therefore, the designer may be forced to accept compromises in either image rejection performance or other design parameters, or both.
Realistically, even in a super-heterodyne receiver circuit with a relatively high IF frequency, there are circuit components that have a finite frequency response in the image frequency band, and some of the image signals will still be down-converted along with the desired signal.